This invention relates to a timing control method used in a magnetic recording and reproducing apparatus such as a hard disk drive, and more particularly to a method capable of extracting servo signals in the sector servo system at a precise timing and capable of effecting recording and reproduction of data signals at a precise timing.
Further, this invention is concerned with an apparatus capable of recording timing signals from which the above-mentioned servo signals can be derived with magnetization inversion phases aligned with each other on a magnetic disk.
In general, as shown in FIG. 1, in a magnetic disk drive, a head arm 3 is rotated about a pivot 4 while revolving a rigid magnetic disk 2 accommodated within a housing 1 at a high speed to move a magnetic head 6 secured to the front end of the head arm 3 through a gimbal arm 5 to a predetermined position of the magnetic disk, thus to record and reproduce various information signals recorded in a form of concentric or helical tracks.
Recently, in the magnetic recording/reproducing apparatus, high density recording systems have been increasingly developed, and consequently higher tracking accuracy is demanded, because of an increase in the track density.
Specifically, there is known a tracking servo system including speed control and positioning control of the magnetic head performed so that the magnetic head is precisely subjected to on-track with respect to a track to be accessed which is formed on the disk surface. See Japanese Laid-open Patent Application No. 242381/1986 previously filed by the applicant of this application. In accordance with this method, a disk-shaped magnetic scale having N-poles and S-poles magnetized alternately at a predetermined pitch on its outer periphery is integrally fitted over the revolving shaft of the access motor for moving the magnetic head in a radial direction of the disk. A sensor using an MR (Magneto-Resistance effect) element is disposed in the vicinity of the outer periphery of the scale to successively sense a position of the magnetic head in the radial direction of the disk, a distance (the number of tracks) between a current position track and a target position track for the magnetic head, to control a moving speed of the magnetic head on the basis of an output from the sensor. In other words, speed control and positioning control of the magnetic head are performed in dependence upon the results sensed.
However, in the tracking servo using such MR element and external magnetic scale, it is difficult to control the magnetic head to converge on the target track. Often an "off track" occurs such that the track center based on the output sensed and the actual track center of the disk do not coincide with each other by changes in temperature, leading to the possibility of failure to correctly read and write intended signals.
To overcome this, a sector servo system as disclosed in the Japanese Laid-open Patent Application No. 246184/1987 filed by the applicant of this application has been proposed, in which servo signals for tracking servo are recorded in advance on a magnetic disk, thus to record a data signal on a desired track or reproduce the recorded data signal therefrom while reproducing the servo signals.
In such a sector servo system, a timing signal S.sub.T shown in FIG. 2 is lacking, and, servo signals are recorded corresponding to both sides of each track center of tracks Tn and Tn+1 and positions of which along the respective track is shifted to each other in a direction of the time base (in a circumferential direction). Recording level of these servo signals is kept constant, so that tracking servo is carried out by moving the magnetic head so as to reproduce these servo signals at equal levels between the two adjacent servo signals on both sides of each track center.
Namely, servo signals S.sub.S and data signals S.sub.D are recorded repeatedly in a direction direction indicated by an arrow A on the magnetic disk, thus to form concentric tracks Tn and Tn+1.
As shown in FIG. 2, the two adjacent servo signals are also shifted in a traverse direction of the track i.e., the direction indicated by an arrow B.
In this example, as shown in FIG. 2, one sector is formed by servo signals S.sub.S and data signal S.sub.D.
In such a sector servo system, the servo signals S.sub.S are extremely important. If the timing for extracting the reproduced output of the servo signal from the reproduced output from the magnetic head has undesirable error, precise tracking could not be conducted.
Since such servo signals S.sub.S are positioned at the head portion of each sector, servo areas where servo signals S.sub.S are recorded and data areas where data signals S.sub.D are recorded are positioned alternately in a direction with respect to the time base.
Accordingly, if the recording timing of the data signals is erroneously shifted, there is the possibility that any servo signal S.sub.S would be erased missing by the over-writing of the data signal S.sub.D.
In such a case, accurate tracking also could not be conducted.
To overcome this, there has been proposed a system to control the timings for recording data signals and extracting servo signals with an output of an FG (frequency generator) or a sensor output of a rotational angle sensor provided on the spindle motor, which spindle motor serves as a reference. Another system has been proposed in which a high frequency signal of which timing is detectable for recording servo signals is recorded in advance along a predetermined track provided on the whole periphery of the data signal recording area using an exclusive clock head provided independently of the read/write head, thus to control the recording timing of the servo signals. The high frequency signal serves as a timing reference.
However, in the case of conducting timing control on the basis of the above-mentioned sensor output, etc., precise timing control is difficult resulting in that the reproduction of servo signals recorded on the disk and the sensor outputs cannot be achieved by eneven thermal expansion of the magnetic disk or of the arm for supporting the magnetic head resulting from changes in temperature, and jitter caused in the waveform shaping circuit.
Further, where a known system in which a timing detectable signal (high frequency signal) recorded on the magnetic disk is employed, there is a problem that it is difficult to discriminate between the timing detectable signal and data signals at the time of movement of the head, failing to effect a secure timing control.
Furthermore, since the clock head is unnecessary after servo signal has been recorded, it is necessary to remove this expensive clock head in the clean room after use. This is not preferable in light of the increased work hours within a clean room.
In addition, space must be provided for the clock head and for its removal. This is not preferable in view of miniaturization of the apparatus.
Further, since the clock head is expensive, it raises the production cost.
There is another system known to the applicant which is shown in FIG. 2, where timing signals (headers) S.sub.T for adjusting starting positions of respective sectors are recorded in a radial direction on the magnetic disk. The timing signals S.sub.T serve as references. Servo signals S.sub.S and data signals S.sub.D are successively recorded in a direction of time base (in a direction indicated by the arrow A).
In a condition where the magnetic disk is stopped at a predetermined angular position, such timing signals S.sub.T are recorded while intermittently feeding the magnetic head by using a linear motor, etc. by a predetermined interval x (x.gtoreq.track width T.sub.W) in the radial direction (in the direction indicated by the arrow B).
Meanwhile, in the case where the above-mentioned timing signals S.sub.T are recorded with an apparatus in which a line l, which connects the center C of the read/write gap of the magnetic head 6 and the pivot 4, and the read/write gap (parallel with the line a in the figure) are perpendicular to each other as shown in FIG. 1, such timing signals S.sub.T are recorded as a magnetization inversion pattern as Shown in FIG. 3.
Thus, where the timing signals S.sub.T having been recorded with such an apparatus are reproduced, reproduced outputs having the same phase can be obtained irrespective of the position of the magnetic head as shown in FIG. 4.
However, when the magnetic head 6 is positioned on the innermost peripheral track and the outermost peripheral track, it is difficult to obtain a stable floating function of the magnetic head 6 resulting from the fact that yaw angle becomes too large, making it difficult to obtain good reproduced signals.
Further, there occurs the problem that the output value (peak level) of the reproduced signal varies in dependence upon the position of the magnetic head 6 with respect to each track as shown in FIGS. 3 and 4.
On the other hand, as shown in FIG. 5, by employing a mechanism such that a line l, which connects the center C of the read/write gap of the magnetic head 6 and the pivot 4, and the read/write gap are caused to have an inclination at an offset angle .theta., it is possible to decrease the yaw angle.
However, in the case of recording timing signals S.sub.T with such an apparatus, timing signals S.sub.T are recorded as a magnetization inversion pattern as shown in FIG. 6, with the result that the waveform of the reproduced output thereof varies depending upon the position of the magnetic head 6 with respect to each track as shown in FIGS. 6 and 7.
Thus, such an apparatus has the problem that an electronic processing circuit for smoothing the waveform is specially required.
A further problem is that since movement in a radial direction of the head for writing timing signals is carried out under the condition where the housing of the magnetic disk apparatus is removed, such a writing operation must be conducted within a clean room, resulting in an increase in assembly cost.